The preparation and use of colloidal materials, such as colloidal silica, are generally knownused. Typically, the silica colloid is first synthesized. The colloid is then coated with a metal oxide. This coating procedure produces both negatively and positively charged surfaces, depending upon the properties of the metallic starting material and the coating method used. Metal-containing silica colloids are useful in various areas, such as chemical mechanical polishing agents in the electronics industry, coating applications, and as support materials in catalytic processes. Despite this versatility, conventional-type silica colloids have several disadvantages.
Liquid suspended metal nanoparticles have several drawbacks in catalysis including difficulties in recycling the nanoparticle catalyst. Immobilization of the catalyst on a solid support makes it possible to recycle the catalyst via simple filtration. Synthesis of the immobilized catalyst (which involves infusing the solid support with metal nanoparticles), however, can be highly time consuming. For example, palladium-catalyzed reaction for carbon-carbon bond formation (Heck Reaction) is a versatile industrial synthesis tool. It includes impregnating a solid support, such as precipitated silica, with Palladium (II) acetate, drying, and reducing with hydrogen gas or other suitable reducing agent.
Two well-known methods of incorporating metals onto porous supports are impregnation (sometimes referred to as the “incipient wetness” technique) and direct synthesis via the sol-gel technique. Impregnation involves taking a solid porous support and adding metal salt solutions to the support. The metal salt solutions intercalate through the structure of the porous support and form surface deposits upon drying. During reduction of the deposited metal salts, the metal particles have a tendency to migrate over the surface of the porous support and must make their way through the porous structure to disperse. This migration leads to sintering and an overall decrease in catalytic activity due to smaller active surface areas (See Hermans & Geus, Interaction of Nickel Ions With Silica Supports During Deposition-Precipitation, Stud. Surf. Sci. Catal., 1979, pp. 113 to 130).
The sol-gel technique (See Lopez et al., Pt/SiO2 Sol-Gel Catalysts: Effects of pH and Platinum Precursor, J. Phys. Chem., 1993, pp. 1671 to 1677) involves mixing metal salts with a silane precursor, such as tetraethyl orthosilicate (“TEOS”). Over time, the TEOS hydrolyzes and its SiOH groups interact with the metal salts. Once the SiOH groups begin to condense and form a precipitate, the metals are dispersed within the SiO2 matrix via terminating SiO− or SiOH groups. The metal salts in the sol-gel technique tend to become occluded within the structure during synthesis.
A need therefore exists for improved methods of incorporating metals into siliceous colloidal compositions. In particular, synthesis methods for creating silica-based colloids or particles having more homogenously dispersed metal loads and enhanced stability over a greater pH range and/or other suitable characteristics are desirable. Furthermore, it is desirable to form such colloids with a narrow particle size distribution.